libressl-EVP_PKEY_CTX_set_tls1_prf_md - TLS PRF key derivation algorithm

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EVP_PKEY_CTX_SET_TLS1_PR(3) Library Functions ManualEVP_PKEY_CTX_SET_TLS1_PR(3)

NAME

EVP_PKEY_CTX_set_tls1_prf_md, EVP_PKEY_CTX_set1_tls1_prf_secret, EVP_PKEY_CTX_add1_tls1_prf_seed — TLS PRF key derivation algorithm

SYNOPSIS

library “libcrypto” #include <openssl/evp.h>
#include <openssl/kdf.h>

int

EVP_PKEY_CTX_set_tls1_prf_md(EVP_PKEY_CTX *pctx, const EVP_MD *md);

int

EVP_PKEY_CTX_set1_tls1_prf_secret(EVP_PKEY_CTX *pctx, unsigned char *sec, int seclen);

int

EVP_PKEY_CTX_add1_tls1_prf_seed(EVP_PKEY_CTX *pctx, unsigned char *seed, int seedlen);

DESCRIPTION

The EVP_PKEY_TLS1_PRF algorithm implements the PRF key derivation function for TLS. It has no associated private key and only implements key derivation using EVP_PKEY_derive(3).

EVP_PKEY_set_tls1_prf_md() sets the message digest associated with the TLS PRF. EVP_md5_sha1(3) is treated as a special case which uses the PRF algorithm using both MD5 and SHA-1 as used in TLS 1.0 and 1.1.

EVP_PKEY_CTX_set_tls1_prf_secret() sets the secret value of the TLS PRF to seclen bytes of the buffer sec. Any existing secret value is replaced and any seed is reset.

EVP_PKEY_CTX_add1_tls1_prf_seed() sets the seed to seedlen bytes of seed. If a seed is already set it is appended to the existing value.

STRING CTRLS

The TLS PRF also supports string based control operations using EVP_PKEY_CTX_ctrl_str(3). The type parameter "md" uses the supplied value as the name of the digest algorithm to use. The type parameters "secret" and "seed" use the supplied value parameter as a secret or seed value. The names "hexsecret" and "hexseed" are similar except they take a hex string which is converted to binary.

NOTES

All these functions are implemented as macros.

A context for the TLS PRF can be obtained by calling:

EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_TLS1_PRF, NULL);

The digest, secret value and seed must be set before a key is derived or an error occurs.

The total length of all seeds cannot exceed 1024 bytes in length: this should be more than enough for any normal use of the TLS PRF.

The output length of the PRF is specified by the length parameter in the EVP_PKEY_derive(3) function. Since the output length is variable, setting the buffer to NULL is not meaningful for the TLS PRF.

RETURN VALUES

All these functions return 1 for success and 0 or a negative value for failure. In particular a return value of -2 indicates the operation is not supported by the public key algorithm.

EXAMPLES

This example derives 10 bytes using SHA-256 with the secret key "secret" and seed value "seed":

EVP_PKEY_CTX *pctx;
unsigned char out[10];
size_t outlen = sizeof(out);

pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_TLS1_PRF, NULL);
if (EVP_PKEY_derive_init(pctx) <= 0)
/* Error */
if (EVP_PKEY_CTX_set_tls1_prf_md(pctx, EVP_sha256()) <= 0)
/* Error */
if (EVP_PKEY_CTX_set1_tls1_prf_secret(pctx, "secret", 6) <= 0)
/* Error */
if (EVP_PKEY_CTX_add1_tls1_prf_seed(pctx, "seed", 4) <= 0)
/* Error */
if (EVP_PKEY_derive(pctx, out, &outlen) <= 0)
/* Error */

SEE ALSO

EVP_PKEY_CTX_ctrl_str(3), EVP_PKEY_CTX_new(3), EVP_PKEY_derive(3)

HISTORY

These functions first appeared in OpenSSL 1.1.0 and have been available since OpenBSD 7.6. GNU June 8, 2025 EVP_PKEY_CTX_SET_TLS1_PRF_MD(3)

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